Removal of pyritic sulfur from coal using solutions containing ferric ions

ABSTRACT

Finely divided coal or solid coal derivatives containing pyrite are reacted with a ferric ion solution; FeCl3 is particularly suitable. The ferric ion is reduced to ferrous ion and free sulfur is formed. The solution is then filtered from the coal which is then washed and heat dried under low pressure. Most of the free sulfur is volatized from the coal due to the heat drying; additional free sulfur can be removed by additional washing and heat drying and/or solvent extraction techniques. At least 60% of the pyrite sulfur and pyrite iron is removed using the process of this invention. If desired, the ferrous chloride can be regenerated; this permits iron oxide to be recovered as a byproduct.

Related US. Application Data Division of Ser. No. 163,893, July 17,1971, Pat. No. 3,768,988, which is a continuation-in-part of Ser. No.116,262, Feb. 17, l97l, abandoned.

US. Cl. 44/2 Int. Cl. C10L 5/24; ClOL 9/00 Field of Search 44/1- R, 1 B,4-6,

References Cited UNITED STATES PATENTS 10/1973 Meyers 44/1 R UnitedStates Patent [1 1 [111 3,917,465 Meyers 5] Nov. 4, 1975 REMOVAL OFPYRITIC SULFUR FROM Primary ExaminerCarl F. Dees Attorney, Agent, orFirmWillie Krawitz; Daniel T. Anderson; Alan D. Akers [57] ABSTRACTFinely divided coal or solid coal derivatives containing pyrite arereacted with a ferric ion solution; FeCl is particularly suitable. Theferric ion is reduced to ferrous ion and free sulfur is formed Thesolution is then filtered from the coal which is then washed and heatdried under low pressure. Most of the free sulfur is volatized from thecoal due to the heat drying; additional free sulfur can be removed byadditional washing and heat drying and/or solvent extraction techniques.At least 60% of the pyrite sulfur and pyrite iron is removed using theprocess of this invention. If desired, the ferrous chloride can beregenerated; this permits iron oxide to be recovered as a byproduct.

1 Claim, 2 Drawing Figures U.S. P816I1t Nov. 4, 1975 Sheet 1 of23,917,465

H2O Q H2O I 2| T 20 I I9 I S S S FecI FeClg WATER SOLUTION PRECIPITATIONT REHEAT BY COOLING YQ R IATM lATM ZIZQF 2|2F I55F T Q Q 22 23 I S J, S

FeCIz F Cl OXIDATION e 3 NR I ATM SOLUTION Fe O IO N S S PYRITE FeCIMAKE-UP filfi gfiQk COAL 2|2F Fig. 1

U.S. Patent Nov. 4, 1975 .Sheet2of2 3,917,465

S WASH H2O FOUR STAGE COAL WA SH \NG SULFUR COOLING MEDIA SULFUR 2CONDENSATION 225F l6 l7 s COAL SULFUR CH DRYING VAPORIZATION l ATM l ATMCOAL 2l2F 450F Fig. 2

REMOVAL OF PYRITIC SULFUR FROM COAL USING SOLUTIONS CONTAINING FERRICIONS This application is a divisional application of application Ser.No. 163,893 filed July 17, 1971, now US. Pat. No. 3,768,988 which is acontinuation-in-part of application Ser. No. 116,262 filed Feb. 17, 1971and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the removal ofpyritic sulfur from coal and solid coal derivatives and morespecifically to the solvent extraction of sulfur from pyrites in coalusing a solution containing a ferric ion.

The present use of coal in the United States is primarily for thepurpose of conversion into electrical energy and thermal generatingplants. One of the principal drawbacks in the use of United States minedcoal is due to their high sulfur contents which can range up to 5%.

Based on a 4% sulfur content, a one million kilowatt plant burns about8500 tons per day of coal and consequently emits 6 tons per day ofsulfur dioxide. If this sulfur could be removed and converted, it wouldproduce 900 tons of H SO daily.

It has long been recognized that in the atmosphere will either retardgrowth or kill vegetation. In addition, the potential hazard to humansappears about the same as for the vegetable kingdom.

While it is possible to remove pyritic sulfur from coal by frothflotation or washing processes; these are not selective so that a largeportion of the coal is discarded along with ash and pyrite. Hence, thesolution so far has been to simply burn coal having a low sulfurcontent. However, many pollution control districts now prohibit the useof coal having an excess of 1% sulfur. The result has been to severelyrestrict the use of many United States coals, 90% of which average about2.5% contained sulfur. This has lead to the importation of low sulfurcontent fuel oils for domestic and industrial use. The crude oilreserves, which are the sources of the residue, are expected to run outin 20 30 years while coal reserves are sufficient for several hundredyears at a minimum.

It is, therefore, an object of this invention to provide a process forthe reduction of sulfur, particularly pyritic sulfur in coal.

Another object is to provide a process for the recovery from coal ofsulfur and sulfur compounds.

Another object is to provide a process for the recovery of iron valuesfrom coal containing pyrite.

Other objects of this invention will become apparent from thedescription and the diagrams to follow.

According to the invention, it has been found that it is possible toreact the pyrite contained in the coal with a solution containing aneffective amount of ferric ion to obtain a high yield of free sulfur. Feion particularly as FeCl is preferred; other ferric salts such asacetate, nitrate, sulfate, citrate, oxide, ferrous ammonium sulfate,etc., may be employed. A typical reaction proceeds as follows:

2 FeCL, FeSJEl E BFeCl 28. The solution containing some free sulfur,ferrous chloride and any unconsumed ferric chloride is removed from thecoal by filtration.

The coal is then washed and dried, preferably by heating in a vacuum;this results in the major portion of free sulfur being volatized asfollows: S.Coal- S 2 coal. If desired, a further wash, filtration andheating will remove more of the sulfur and any residual ferrous ion. Oneor more extractions with a suitable organic sulfur solvent such asbenzene, kerosene or para cresol may be employed to further reduce thesulfur content of the coal.

Regeneration of the unused ferric chloride and ferrous chloride solutionmay be accomplished by first concentrating the solution by evaporatingmost of the water. The concentrated solution is cooled, therebyprecipitating the ferrous chloride from the ferric ch10 ride, most ofthe latter still remaining in solution. The precipitated ferrouschloride is air oxidized to ferric chloride and iron oxide and finallythe ferric chloride is recycled and the iron oxide recovered.

Typical treatment temperatures may vary from 50 C. Reflux times aretypically /2 2 hours and higher. Typical cool particle sizes may varyfrom -200 mesh to /2-inch pieces. Atmospheric pressure may be employed,but higher pressures can also be used.

The effective amount of the ferric ion solution employed for extractiondepends on the amount of coal treated and its pyritic sulfur content,the amount of sul fur desired to be extracted, extraction times,extraction temperatures, concentration of the ferric ion in thesolution, etc.

The reaction of ferric chloride and ferrous persulfide to produce freesulfur is known. However, it was unexpected that the reaction withferric ion (e.g., FeCl and pyrite could be carried out in a coal mediumsince pyrite is dispersed very finely throughout the coal matrix, andpenetration of such an organic matrix with water is known to bedifficult. Furthermore, the volatization of sulfur from coal is unusualsince it well might be expected that the free sulfur would recombineeither with iron or with the coal upon heating. It is also well knownthat iron pyrites may be oxidatively dissolved from the coal matrix withstrong aqueous oxidizing agents such as NHO H 0 or HOCl. This willconvert the sulfur content to sulfate, but not to free sulfur. This isthe basis for chemical analysis of the pyritic sulfur content of coal;however, such strong oxidizing agents also extensively oxidize theorganic coal matrix. By contrast, ferric salts are almost totallyselective in the sense that the organic coal matrix is undisturbed.Hence, ferric salts, but not I-INO H 0 or HOCl, provide an economicalroute to the removal of pyrites from coal.

Coals which may be employed in this invention include those which areconsidered as coals in the popular or commercial sense, such asanthracites, charcoal, coke, bituminous coals, lignites, etc. Inaddition, chars from hydrocracked coals and middlings are all capable ofbeing refined by the extraction process of this invention.

The invention will be understood by reference to FIGS. 1 and 2 in whichferric chloride make-up solution and coal are fed into a pyrite reactor10 maintained at atmospheric pressure and about 212F. Pyrite (FeS isextracted from the coal, and the slurry containing unreacted ferricchloride, ferrous chloride, sulfur, ferrous persulfide, and the treatedcoal are fed to a coal filtration unit 11. Vacuum disk filters in thecoal filtration unit are used to separate the bulk of the iron chloridesolution from the treated coal.

In the coal washing sections 12, 13, 14, and 15, four stages ofcountercurrent washing with intermediate filtration steps are used toreduce the residual chloride 3 content of the coal to less than about100 ppm. A suitable residence time of the coal in each of the washingstages is about 15 minutes; rotary vacuum disk filters are used toseparate the coal and wash the solution between washing stages.

The washed coal is then fed to a coal drying unit 16 where rotary steamtube dryers are employed to re move the residual water from the washedcoal, this operation being carried out at atmospheric pressure and about212F. The heated dry coal is then forwarded to a sulfur vaporizationunit 17 where free sulfur, which was produced in the extraction reactionin reactor 10, is vaporized at atmospheric pressure and a temperature ofabout 450F or under reduced pressure (30 min.) and at 250 350F. Thevaporized sulfur is removed by nitrogen gas into a sulfur condensationunit 18 and cooled to about 225F causing it to condense. The sulfurvapor is then passed to a recovery unit as bright sulfur. The treatedcoal was reduced pyrite content is then forwarded for use.

In the ferric chloride regeneration stage, the filtrate the insolubleiron oxide by dissolving in water. The ferric chloride solution isrecycled to the ferric chloride make-up solution for use in reactor 10.The iron oxide is filtered from the ferric chloride solution and may berecovered as a byproduct of the process.

Typical coals which may be employed in the process include Missour,Lower Freeport, Bevier, Indiana No. V, and Pittsburgh. These coalscontain sulfur forms as shown in Table 1 when freshly mined. As theystand exposed to air, small amounts of sulfate sulfur are formed fromthe pyrite content.

from the coal filtration unit 11 is passed to a thickener unit wherewater is vaporized from the solution at atmospheric pressure and about212F. The concentrated solution is then passed to a precipitation unit20 where ferrous chloride is precipitated by cooling the solution to155F at atmospheric pressure. Unreacted ferric chloride solution fromthe precipitation unit 20 is heated in a reheater 21 and then combinedwith ferric Table 2 shows the original pyritic sulfur content of theMissouri and Lower Freeport coals and the reduction in sulfur contentdueto treatment of FeCl;,.

It will be observed that a marked reduction in pyritic sulfur occursafter only a single treatment with FeCl followed by a water washing anddrying.

Table 3 shows the effect of employing an organic solvent to remove thefree sulfur which remains following chloride make-up for feeding to thereactor 10. the FeCl and water washing treatment.

TABLE 2 FeCl Extraction Data Wt Loss Reflux After Sam- Wt Mol- (90CWashing Sulfur Pyritic ple Coal Vol. arity 2Fe+3 Time & Drying Eschka Wt'71 Fe Sulfur Coal No. Grms FeCL, FeCl Pyritic Fe (hrs.) Wt 71 Wt 7c inCoal Removed Missouri Untreated 4.75 1.65

(Mesh Size -200) Missouri 1 40 200 0.5 3.9/1 16 0.22 (Mesh Size 20())0.17

Missouri 2 500 0.3 5/1 2 9.1 0.06 3 1% (Mesh Size 200) 0.84

Missouri 3 30 500 0.5 13/1 20 +2.0 3.67 0.19 62'7r (Mesh Size 2(J())3.65 0.19

Lower Freeport U. 3.54 (Mesh Size l4) 3.99 3 5 Lower Freeport 4 50 7300.5 6/1 2 -l4.l* 1.99 I 1.10 -7r (Mesh Size 14) 'ASTM D271 '-Bureau ofMines procedure and Standard Methods of Chemical Analysis. Furman.Volume 1. page 542. Remarks: Residue removed from condenser was analyzedby electron microprohe as follows: Fe. S. Si. O. C were majorconstituents: Ca. Cl. Al were trace- Yellmv Crystals formed: Hg spottest for free S was positive.

"Sample 2 was washed once with 250 cc hot water and dried 24 hours in aC vacuum oven. Samples 3 and 4 were washed twice with 250 cc hot waterand dried 72 hours at 90C in a vacuum oven.

TABLE 3 SOLVENT EXTRACTION DATA FOLLOWING FeCl TREATMENT AND WATER WASHWt. Sulfur Concentration Sample Extraction Loss. /1 Extrac- No. SolventProcedure Gain Eschka tion 1 Benzene 5-10 3 hot (80C) 3.22 8471 min, 100cc washings 3.21 2 Benzene 5l0 3 hot (80C) 3.62 60% min. 100 cc washings3.61 3a Benzene 5l() 3 hot (80C) +2 3.06 897:

min. 100 cc washings 3.03 3h p-Cresol 5-10 V2 hr (200C) l 3.67! 2.8l l

min. reflux 2.18 4 Benzene 5-10 3 hot (80C) 2.23 74% min. 100 ccwashings 1.77

TABLE 4-continued PYRlTlC SULFUR REMOVAL DATA Pyritic Total Total TotalSulfur Sulfur Sulfur Sulfur Coal" Removed Removed Before AfterPittsburgh 39 78 1.8] 1.10

From the data in Table 3, it appears that the organic solvent treatmentcauses a major portion of the pyritic sulfur to be extracted; also, useof para cresol appears to result in extraction of organic as well aspyritic bound sulfur. While the efficiencies shown range from 60% to atleast 89%, this efficiency range can be changed by altering such factorsas wash times, particle size, amounts and concentrations of FeClandsolvents, ferric salt treatment, reflux temperature, etc.

Table 4 shows the effect of FeCl extraction on various coals employingreaction conditions similar to Table 2. The table shows that 72 93% ofthe pyritic sulfur content may be removed in two hours by 0.5 M FeClsolution from a wide variety of coals. Further, the process wasapplicable to all the coals, and in the case of Indiana No. V, theextraction efficiency was excellent.

TABLE 4 PYRlTlC SULFUR REMOVAL DATA Pyritic Total Total Total SulfurSulfur Sulfur Sulfur 5 Coal" Removed Removed Before After Lower Freeport48 75 3.87 2.01

Lower Freeport 64 72 3.40 I 21 Bevier 36 72 4.60 2.94 Indiana No. V 5193 3.28 1.67 5() "All coals were "l4 mesh except Bevier which was -200mesh The process of this invention is extremely efficient in that atleast 60% of the pyrite sulfur is extracted and the iron employed forextraction is easily recovered (about -90%) and may be reused.Furthermore, iron removal is facilitated since the iron contained in theFeClextraction solution and the iron in the pyrite areindistinguishable; hence, no special techniques are required to separatedifferent metals from the wash extraction operation if metal recyclingis desired.

In addition, the process is simple in that no high temperatures,pressures or catalysts are required.

Finally, the extraction with FeCl does not produce an interaction withthe organic coal matrix; this permits substantially all of the coal tobe utilized as low sulfur fuel.

What is claimed is:

1. An apparatus for reducing the pyritic sulfur content of coalcomprising in combination:

means adapted to react coal containing iron pyrite with a solution offerric ion;

washing means for removing reactant solution from the coal; and

means for removing from the coal, free sulfur formed by the reactionbetween the pyrite and ferric ion.

* l l =l

1. AN APPARATUS FOR REDUCING THE PYRITIC SULFUR CONTENT OF COALCOMPRISING IN COMBINATION: MEANS ADAPTED TO REACT COAL CONTAINING IRNPYRITE WITH A SOLUTION FOR FERRIC ION, WASHING MEANS FOR REMOVINGREACTANT SOLUTION FROM THE COAL, AND MEANS FOR REMOVING FROM THE COAL,FREE SULFUR FORMED BY THE REACTION BETWEEN THE PYRITE AND FERRIC ION.